Currently, a glass substrate is mainly used as a substrate in the field of electronic devices such as a flat panel display and electronic paper. However, a glass substrate is disadvantageously heavy and brittle. Thus, a glass substrate is not always an ideal substrate. On this account, studies have been actively made so as to provide a flexible device in which a substrate made of glass is replaced by a substrate made of a polymer material. However, many of techniques for providing such a flexible device require new production techniques and apparatuses. Accordingly, a flexible device in which a polymer material is used has not yet been mass-produced.
Meanwhile, recently, there has been proposed, as a shortcut to efficient mass production of flexible devices, a method for producing a flexible device in accordance with an ordinary process for producing a glass substrate and by use of a laminate in which a polyimide resin layer is provided on a glass substrate. According to the process in which the laminate is used, the flexible device is obtained by separating the polyimide resin layer from the glass substrate in a final step.
In the process, the laminate is required to have smoothness and low warpage so as to be favorably handled. In other words, the polyimide film layer of the laminate is required to have a sufficiently smooth surface and a coefficient of linear thermal expansion which coefficient is substantially equal to that of glass. Note that soda-lime glass and alkali-free glass, each of which is generally used for a glass substrate, have respective coefficients of linear thermal expansion of approximately 8 ppm/° C. to 9 ppm/° C. and approximately 3 ppm/° C. to 5 ppm/° C. Further, a processing temperature during production of an amorphous silicon thin film transistor reaches 300° C. to 350° C. at maximum. A material suitable for such a process is inevitably limited. This is because a coefficient of linear thermal expansion of a general polyimide is greater than that of glass. For example, Patent Literature 1 discloses a method in which a laminate is obtained by (i) casting, on an inorganic substrate, a solution of a polyimide precursor obtained from (a) 3,3′,4,4′-biphenyltetracarboxylic dianhydride and (b) paraphenylene diamine, 4,4″-diaminoparaterphenyl, and the like and (ii) subjecting the solution to thermal imidization.
Patent Literature 2 discloses a polyimide precursor composition in which a solvent that has a boiling point of 160° C. or lower and is useful for production of electrical and electronic materials such as a semiconductor device and a multilayer circuit board is dissolved in a solvent that accounts for 30% or more of a total solvent weight. The polyimide precursor composition contains a surfactant so that nonuniformity in thickness less occurs in a coated film. The polyimide precursor composition also contains an organosilicon compound as an adhesive assistant so that a heat-treated polyimide coated film and a substrate more strongly adhere to each other.
Patent Literature 3 discloses an alkoxysilane-modified polyamic acid solution obtained by reacting an amino group-containing alkoxysilane compound with a polyamic acid in a solution. Patent Literature 3 also discloses (i) that the polyamic acid is obtained by reaction between an aromatic diamine and an aromatic tetracarboxylic dianhydride and (ii) that a molar ratio obtained by dividing a total number of moles of the aromatic tetracarboxylic dianhydride by a total number of moles of the aromatic diamine falls within a specific range.